1. Field of the Invention
The invention relates generally to the field of oil and gas production. More specifically, the present invention relates to a method of producing a shaped charge liner from an injection molding process.
2. Description of Related Art
Perforating guns are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore, and the casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore.
Shaped charges known in the art for perforating wellbores are used in conjunction with a perforation gun. One embodiment of a traditional shaped charge 5 is illustrated in FIG. 1. As shown, shaped charge 5 includes a housing 6, a liner 10, and a quantity of high explosive 8 inserted between the liner 10 and the housing 8 where the high explosive 8 is usually HMX, RDX PYX, or HNS. When the high explosive 8 is detonated, the force of the detonation collapses the liner 10 and ejects it from one end of the charge at very high velocity in a pattern called a “jet”. The jet penetrates the casing, the cement and a quantity of the formation.
Some of the traditional methods of producing shaped charge liners include sintering and cold working. Cold working involves mixing a powdered metal mix in a die and compressing the mixture under high pressure into a shaped liner. Typically, these liners comprise a composite of two or more different metals, where at least one of the powdered metals is a heavy or higher density metal, and at least one of the powdered metals acts as a binder or matrix to bind the heavy or higher density metal. Examples of heavy or higher density metals used in the past to form liners for shaped charges have included tungsten, hafnium, copper, or bismuth. Typically the binders or matrix metals used comprise powdered lead, however powdered bismuth has been used as a binder or matrix metal. While lead and bismuth are more typically used as the binder or matrix material for the powdered metal binder, other metals having high ductility and malleability can be used for the binder or matrix metal. Other metals which have high ductility and malleability and are suitable for use as a binder or matrix metal comprise zinc, tin, uranium, silver, gold, antimony, cobalt, copper, zinc alloys, tin alloys, nickel, and palladium.
One of the problems associated with cold working a liner is a product having inconsistent densities. This is usually caused by migration of either the binder or the heavy metal to a region thereby producing a localized density variation. A lack of density homogeneity curves the path of the shaped charge jet that in turn shortens the length of the resulting perforation. This is an unwanted result since shorter perforations diminish hydrocarbon production. Moreover, cold worked liners have a limited shelf life since they are susceptible to shrinkage thereby allowing gaps to formed between the liners and the casing in which they are housed. These liners also tend to be somewhat brittle which leads to a fragile product.
Sintered liners necessarily involve a heating step of the liner, wherein the applied heating raises the liner temperature above the melting point of one or more of the liner constituents. The melted or softened constituent is typically what is known as the binder. During the sintering step, which is typically performed in a furnace, the metal powders coalesce while their respective grains increase in size. The sintering time and temperature will depend on what metals are being sintered.
The sintering process thus forms crystal grains thereby increasing the final product density while lowering the porosity. Typically sintering is performed in an environment void of oxygen or in a vacuum. However the ambient composition within a sintering furnace may change during the process, for example the initial stages of the process may be performed within a vacuum, with an inert gas added later. Moreover, the sintering temperature may be adjusted during the process, wherein the temperature may be raised or lowered during sintering.
Prior to the sintering step the liner components can be cold worked as described above, injection molded, or otherwise formed into a unitary body. However the overall dimensions of a sintered liner can change up to 20% from before to after the sintering step. Because this size change can be difficult to predict or model, consistently producing sintered shaped charge liners that lie within dimensional tolerances can be challenging. Information relevant to shaped charge liners formed with powdered metals is addressed in Werner et al., U.S. Pat. No. 5,221,808, Werner et al., U.S. Pat. No. 5,413,048, Leidel, U.S. Pat. No. 5,814,758, Held et al. U.S. Pat. No. 4,613,370, Reese et al., U.S. Pat. No. 5,656,791, and Reese et al., U.S. Pat. No. 5,567,906.
Therefore, there exists a need for a method of consistently manufacturing shaped charge liners, wherein the resulting liners have a homogenous density, have consistent properties between liner lots, have a long shelf life, and are resistant to cracking.